There has been devised and demonstrated a system for controlling the temperature of water to be fed into a water cooling tower in which the rotational speed of a fan motor is so controlled that the power consumption of the fan motor is decreased and consequently the energy can be saved. For instance, in FIG. 1 is shown a prior art system for controlling the temperature of water to be fed into a water cooling tower which is disclosed in Japanese Patent Application laid open for public inspection under No. 143398/1980. The system comprises a water cooling tower 1, a plurality of fans 2, a plurality of single speed motors 3 for driving the fans 2, a frequency converter 5 whose output is connected in parallel with a power supply 4 for the single speed motors 3 and whose input is connected to a power supply 4, means 6 for detecting the load conditions of the water cooling tower 1 and a control device 7 for controlling the frequency converter 5. In response to the load condition signals (which represent, for instance, the temperature of returned water, the temperature difference, the temperature of water to be fed into the water cooling tower and the wet bulb temperature at a suction port) detected by the detecting means 6, the control device 7 generates a control signal in response to which the rotational speed of the single speed motors 3 is determined. In response to the control signal from the control device 7, the single speed motors 3 are turned on (that is, the motor 3 is rotated at 100% or at a rated rotational speed), turned off (stopped) or rotated at a predetermined rotational speed (for instance at 50% of the rated speed). When the single speed motors 3 are rotated at 100%, the power is supplied from the power supply 4; but when the single speed motors 3 are rotated at 50%, the power is supplied from the frequency converter 5. Because of the frequency converter 5, the single speed motors 3 can be rotated at any desired speed. However, since the power is proportional to the cube of a rotational speed of the single speed motor 3, when the frequency conversion rate is 50%, the theoretical power can be decreased to one eighth, whereby the energy can be saved. When the single speed motors 3 are rotated at 50% by the frequency converter 5, the cooling tower characteristic curves are shown in FIG. 2. The temperature T of water to be fed into the water cooling tower is plotted along the ordinate while the wet bulb temperature T.sub.wo, along the abscissa. White dots represent the 100% operation; half-black dots represent the 50% operation; and black dots represent that the single speed motors are turned off. When the four fans 2 are operated at their full capacity, the performance characteristic curve is indicated by the rightmost curve. The curve next to the rightmost curve indicates the performance when one of the fans 2 is operated at its half capacity. As the capacity of the fan 2 is decreased, the curves as shown at the left portion can be obtained. When the single speed motors 3 are controlled in speed stepwise in response to the load conditions, the performance curves can be changed so that a desired set condition, that is, the temperature T.sub.o of water to be fed into the water cooling tower can be attained and maintained.
The above described water temperature control method is adapted for use with a large-sized water cooling tower because a plurality of single speed motors 3 can be driven by a single frequency converter 5, but, as described above, the single speed motors 3 are controlled stepwise; that is, they are operated at 100%, 50% and 0% (stopped) so that, as is clear from FIG. 2, the temperature T.sub.o of water to be fed into the water cooling tower cannot be controlled with a desired degree of precision. Therefore, the above-described water temperature control method is not adapted for used with a small-sized water cooling tower in which the temperature of water to be fed into the tower must be controlled with a high degree of precision.
In order to solve this problem, there has been devised and demonstrated a water temperature control system of the type as shown in FIG. 3 which is adapted for use with a small-sized water cooling tower. In this system,the control devices 7 and the frequency converters 5 are equal in number to the single speed A.C. motors 3. Each frequency converter 5 has the power capacity sufficient enough to drive the corresponding single speed motor 3 at 100% (that is, at its rated speed). In other words, the frequency converter 5 can continuously change the rotational speed of the corresponding single speed motors 3 from 50% to 100%.
With this system, the rotational speeds of the single speed A.C. motors 3 can be continuously controlled as described above so that the temperature T.sub.o of the water to be fed into a water cooling tower can be controlled with a high degree of accuracy, but there exists a disadvantage that each single speed motor 3 must be provided with one frequency converter 5. There is a further disadvantage that the power capacity of the frequency converter 5 becomes high because the frequency converter 5 must drive its corresponding single speed motor 3 at 100%. As a result, the above-described water temperature control system is adapted for use with a small-sized water cooling tower, but its capital cost is expensive.
That is, according to the prior art water temperature control methods, in order to decrease the electric power consumption and to save the energy, the accuracy with which the temperature of water to be fed into a water cooling tower must be sacrificed. On the other hand, in order to attain a high degree of accuracy in control so as to stabilize the temperature of water to be fed into a water cooling tower, the power consumption is increased and the system is also increased in size. So far such dilemma has not been satisfactorily solved.
The present invention was made in order to substantially solve the above and other problems encountered in the prior art water temperature control methods and has for its object to provide a method for controlling the temperature of water to be fed into a water cooling tower in which when a great power is needed, some motors are driven by a constant-frequency power supply while the remaining motors are driven by frequency converters so that the dilemma encountered in the convertional water temperature control methods that the power consumption is increased in the continuous control can be solved and the temperature of water to be fed into a water cooling tower can be controlled with a high degree of accuracy while the electric energy can be saved.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.